Since inkjet recording methods have advantages over other image forming methods such that color images can be easily formed; and running costs are low, inkjet recording methods have been broadly used recently. However, inkjet recording methods have a drawback such that defective images (such as blurred images including blurred character images) are easily formed depending on combinations of the ink and the recording medium used.
For example, when coated papers which are used for commercial printing and publication printing and in which a filler such as calcium carbonate and kaolin is used for the coat layer, are used as recording media, problems such that images are seriously blurred, and image density is seriously low are caused. The reasons for the problems are considered as follows. Specifically, since such coated papers cannot absorb a large amount of ink in a short time unlike inkjet recording papers, part of the ink, which is not well absorbed, is spread in the horizontal direction, thereby forming a blurred image. In addition, even when the ink penetrates into the coat layer of coated papers, the filler (such as kaolin) in the coat layer opacifies the penetrated ink, thereby forming a low density image. Therefore, such coated papers as used for commercial printing and publication printing have been considered to be entirely unsuitable for inkjet printing.
As for inkjet inks, aqueous pigment inks in which fine pigment particles are dispersed in water attract attention. Since pigments used for inkjet inks are similar in composition to colorants of inks used for commercial printing, it is expected that images having feeling similar to that of images formed by commercial printing can be formed by inkjet inks. However, even when images are formed on coated papers used for commercial printing and publication printing by inkjet printing methods using pigment inkjet inks, the blurred image problem and/or other problems such that the pigments are not fixed to the coated papers, and glossy images cannot be formed tend to be caused.
In attempting to solve the problems, an inkjet recording method (a first method) using a combination of a pigment ink having high penetrating property and a recording medium having low ink absorbability is proposed. In addition, another method (second method) in which a heat roller is used as a drying assisting device to rapidly dry ink images is proposed to improve the drying property and fixing property of the ink images. Further, another method (third method) in which a protective layer is formed on inkjet images using a UV varnish to protect the ink images, i.e., to prevent occurrence of a problem in that the pigment included in the ink images and remaining on the surface of a recording paper is scraped off when the images are rubbed, is proposed.
In commercial printing and publication printing, hundreds or thousands sheets of prints are generally produced in one printing operation. Therefore, printing machines are required to stably produce prints without forming defective images. When inkjet printers are used for such commercial printing and publication printing, there is a risk for a problem in that inkjet nozzles are clogged with dried inks (i.e., nozzle clogging problem), or inks are ejected in wrong directions due to fixation of dried inks around the exits of inkjet nozzles, thereby forming images having white lines or undesired line images. The best method for preventing occurrence of the problem is to add a water-soluble organic solvent having a high boiling point to an inkjet ink to impart a relatively low drying property to the ink.
However, when the above-mentioned first method is used while using a combination of an inkjet ink including a water-soluble organic solvent having a high boiling point (to prevent occurrence of the nozzle clogging problem), and a recording medium such as coated papers for use in commercial printing, which has an extremely low ink absorbing property, another problem in that it takes time before the ink is fixed to the recording medium to an extent such that the ink image is not blurred even when rubbed is caused. Therefore, when the first method is used for commercial printing, it takes time before images are fixed to recording media, thereby causing a problem in that the resultant prints cannot be handed out as leaflets or catalogs soon after the prints are formed.
In the above-mentioned second method, an oil-based ink including an aliphatic hydrocarbon as a main component is used. Therefore, when the ink is heated, an organic compound is evaporated and discharged from the printer, thereby causing environmental pollution.
In the above-mentioned third method, the printer has to have a heater for forming images. In addition, in the aftertreatment section of the printer, a UV varnish is ejected toward the entire surface of a recording medium, and the UV varnish adhered to the recording medium is crosslinked by a device such as UV lamps. Therefore, the printer has a complex structure and is not environmental friendly.
For these reasons, the inventors recognized that there is a need for an image forming method by which high quality full color images having good drying property and rub resistance can be formed on papers for use in commercial printing at a high speed.